Because the marketing department found out that you can get an amp to deliver a much higher number of watts, if even for an extremely brief, unusable period. Bigger number = better marketing.
I think the customers discovered it first by loading down 4 ohm amps to 2 ohms and discovering how smart they (thought they) were.
Also, the number of watts an amp can deliver will depend on the frequency that you're working with.
Generally solid state amps make the same max power at all frequencies, but there can be a subtle difference in clipping power during continuous sinewave testing, for sine waves that are lower frequency that the PS refresh rate (120Hz). When testing at significantly higher sine wave frequencies than the PS charging rate, the draw is roughly 50% from each rail, but at lower frequencies the draw can be effectively from only one rail at a time (for 2x the capacitor sag). Increasing the size of the reservoir cap, a cost in power amps can mitigate this rail sag.
Again, some companies will pick the frequency that gives the biggest number to make their amp look better, even if that frequency is not the way the amp would be used.
A tell for amps with marginal reservoir cap size is higher rated output power at 1kHz than 20-20kHz (for same distortion, but they rarely rate them apples to apples). Music is not generally continuous sine waves, so don't expect a huge real world difference from oversized PS caps.
So, unless there's an absolute standard that every manufacturer followed, just saying XX watts at X ohms with X% distortion will not give you an accurate picture to compare amps.
In life we don't always get neat answers... I used to be product manager for power amps, so my day job was writing such specs.. I believe most companies try to be honest, but customers do reward specmanship (unfortunately).
JR
